Aerodynamics Matter !!

Ron-C wrote:

I suspect that the back shape has more influence on the drag force than the front although everyone seems to focus on making the front nice and round. Your TV has already punched the hole. It is more likely the back creating suction with the wall behind you.

Ever see an airplane or fish with a flat tail section?

Surprisingly, the front and rear of the trailer generate just about the same amount of drag. It's true that the truck does "punch the hole", however there's a low pressure zone that forms between the truck and trailer. Air that would normally follow the top, bottom, and sides of the tow vehicle, rushes into this area. The air coming from all different directions causes quite a bit of turbulence which translate into drag.

This is one of the reasons why air deflectors aren't very effective in most situations. The air comes across the deflector, but then falls off the edge before it can reach the top of the trailer. The air then falls into the trailer gap where it becomes turbulent air.

Studies on the aerodynamics of big rigs show that about 20 percent of the drag experienced by the combination comes from the gap between the tractor and the trailer. The rear of the unit generates about 25 percent of the drag. Big rigs today now come with extended fairings on the back of the cab to minimize the amount of drag experienced in this area.

Keep in mind this was all done with a TV and trailer that's pretty equally matched. In our world the combinations aren't normally matched so well. With an large SUV you're looking at a 6.5 ft tall and wide tow vehicle pulling a 8ft wide and 11 ft tall trailer with a trailer gap of 3' to 5'. It's even worse for a pickup because the air falls off the cab.